The present invention relates generally to paper products. More particularly, the invention concerns methods for applying chemical additives to cellulosic fibers and the paper products that can be obtained by the methods.
In the manufacture of paper products, it is often desirable to enhance physical and/or optical properties by the addition of chemical additives. Examples of properties that are developed or enhanced through the addition of chemical additives include but are not limited to dry strength, wet strength, softness, absorbency, opacity, brightness and color. During the papermaking process, chemical additives are commonly added to fiber slurries in the wet end, before the fibers are formed into a web, dewatered and dried. Traditionally, wet end additives are added to a fiber slurry that is between 0.5 and 5 percent consistency. The slurry may then be further diluted in the papermaking process before a final dilution at the fan pump to the ultimate forming consistency.
Wet end chemical addition has several advantages over topical spray, printing or size press chemical addition methods. For instance, wet end chemical addition provides a uniform distribution of chemical additives on the fiber surfaces. Additionally, wet end chemical addition allows a selected fiber fraction to be treated with a specific chemical additive in order to enhance the performance of the paper and/or the effectiveness of the chemical additive. Further, wet end chemical addition enables multiple chemistries to be added to a fiber slurry, either simultaneously or sequentially, prior to formation of the paper web.
One difficulty associated with wet end chemical addition is that the water soluble or water dispersible chemical additives are suspended in water and are not completely adsorbed onto the cellulosic fibers. To improve adsorption of wet end additives, chemical additives are often modified with functional groups to impart an electrical charge when in water. The electrokinetic attraction between charged additives and the anionically charged fiber surfaces aids in the deposition and retention of chemical additives onto the fibers. Nevertheless, the amount of chemical additive that can be retained in the wet end generally follows an adsorption curve exhibiting diminishing effectiveness, similar to that described by Langmuir. As a result, the adsorption of water soluble or water dispersible chemical additives may be significantly less than 100 percent, particularly when trying to achieve high chemical additive loading levels.
Consequently, at any chemical addition level, and particularly at high addition levels, only a fraction of the chemical additive is retained on the fiber surface. The remaining fraction of the chemical additive remains dissolved or dispersed in the suspending water phase. These unadsorbed chemical additives can cause a number of problems in the papermaking process. The exact nature of the chemical additive will determine the specific problems that may arise, but a partial list of problems that may result from unadsorbed chemical additives includes: foam, deposits, contamination of other fiber streams, poor fiber retention on the machine, compromised chemical layer purity in multilayer products, dissolved solids build-up in the water system, interactions with other process chemicals, felt or fabric plugging, excessive adhesion or release on dryer surfaces, physical property variability in the finished product, and the like.
Therefore, what is lacking and needed in the art is a method for applying adsorbable chemical additives, particularly a dye, onto cellulosic fiber surfaces in the wet end of the papermaking process such that the amount of unadsorbed chemical additives in the process water is reduced or eliminated. The method minimizes the associated manufacturing and finished product quality problems that would otherwise occur.
It has now been discovered that chemical additives can be adsorbed onto cellulosic papermaking fibers at high levels with a minimal amount of unadsorbed chemical additives present in the papermaking process water. This is accomplished by treating a fiber slurry with an excess of the chemical additive, allowing sufficient residence time for adsorption to occur, filtering the slurry to remove unadsorbed chemical additives, and redispersing the filtered pulp with fresh water. Because the filtrate from the thickening process contains unadsorbed chemical additive, it is not sent forward in the process with the chemically treated fibers. Rather, the filtrate may be sent to the sewer or reused in a processing step prior to the filtration step.
Hence in one aspect, the invention resides in a method for applying chemical additives to cellulosic fibers. The method comprises the steps of: creating a fiber slurry comprising water, cellulosic fibers, and an adsorbable chemical additive; dewatering the fiber slurry to remove unadsorbed chemical additive; and redispersing the fibers with fresh water. This method for processing cellulosic papermaking fibers enables chemical additives to be adsorbed by fibers while at the same time maintaining significantly lower levels of unadsorbed chemical additive in the water phase compared to traditional wet end chemical addition. Thus, higher concentrations of the chemical additive on the fiber relative to the process water can be achieved as compared to what has been possible with prior methods.
For purposes of the present invention, the term xe2x80x9ccellulosicxe2x80x9d refers to papermaking fibers comprising an amorphous carbohydrate polymer, in contrast to synthetic fibers. The term xe2x80x9cadsorbablexe2x80x9d is used herein to refer to a chemical additive that can be assimilated by the surface of a cellulosic fiber, in the absence of any chemical reaction involving the chemical additive and the cellulosic fiber. The term xe2x80x9cunadsorbedxe2x80x9d refers to any portion of the chemical additive that is not adsorbed by the fiber and thus remains suspended in the process water. The term xe2x80x9cfresh waterxe2x80x9d is used herein to refer to water that is substantially free of the unadsorbed chemical additive. Most desirably, the fresh water is completely free of the chemical additive.
The fiber slurry is desirably dewatered to increase the consistency of the fiber slurry to about 20 percent or greater, and particularly to about 30 percent or greater, in order to remove the majority of the water containing the unadsorbed chemical additive. The fibers are thereafter redispersed, desirably to decrease the consistency of the fiber slurry to a level suitable for papermaking, to about 20 percent or less, and more particularly to about 5 percent or less, such as about 3 to about 5 percent.
The present method allows for the production of fiber furnishes that are useful for making paper products, and particularly layered paper products. Thus, another aspect of the invention resides in a fiber furnish that has a higher chemical additive loading than could otherwise be achieved in combination with the relatively low level of unadsorbed chemical additive in the water. This is because chemical additive loading via traditional wet end addition is often limited by the level of unadsorbed chemical and its associated processing difficulties such as foam, deposits, chemical interactions, felt plugging, excessive dryer adhesion or release or a variety of paper physical property control issues caused by the presence of unadsorbed chemical in the water.
In one embodiment, a fiber furnish of the present invention comprises water, cellulosic fibers, and an adsorbable chemical additive. The amount of chemical additive adsorbed onto the fibers is about 2 kilograms per metric ton or greater, and the amount of unadsorbed chemical additive in the water is between 0 and about 20 percent of the amount of chemical additive adsorbed onto the fibers. In particularly desirable embodiments, the amount of adsorbed chemical additive is about 3 kg/metric ton or greater, particularly about 4 kg/metric ton or greater, and more particularly about 5 kg/metric ton or greater. Moreover, the amount of unadsorbed chemical additive in the water is between 0 and about 15 percent, particularly between 0 and about 10 percent, and more particularly between 0 and about 7 percent, of the amount of adsorbed chemical additive.
When the chemical additive is a dye, the amount of dye adsorbed onto the fibers can vary from between about 0.01 to about 20 kg per metric ton. Preferably, the amount of dye adsorbed onto the fibers is from between about 0.05 to about 15 kg per metric ton. More preferably, the amount of dye adsorbed onto the fibers is from between about 0.05 to about 7.5 kg per metric ton. Even more preferably, the amount of dye adsorbed onto the fibers is from between about 0.05 to about 10 kg per metric ton. Most preferably, the amount of dye adsorbed onto the fibers is from between about 0.05 to about 2.0 kg per metric ton.
The amount of unadsorbed dye in the water can vary from between 0 and about 20 percent of the amount of dye adsorbed onto the fibers. More preferably, the amount of unadsorbed dye in the water can vary from between about 5 to about 20 percent of the amount of dye adsorbed onto the fibers. Moreover, the amount of unadsorbed dye in the water is from between 0 and about 15 percent, particularly from between 0 and about 10 percent, and more particularly, from between 0 and about 7 percent of the amount of unadsorbed dye.
Another aspect of the invention resides in a method for making chemically treated paper products. The method includes the steps of: creating a first fiber slurry containing water, cellulosic fibers, and an adsorbable dye, and creating a second fiber slurry that is substantially free of the adsorbable dye. The first fiber slurry is dewatered to remove unadsorbed dye before the fibers in the first fiber slurry are redispersed with fresh water. The first and second fiber slurries are then used to form a paper product using a layered headbox. The first fiber slurry is supplied to a first layer of the headbox and the second fiber slurry is supplied to a second layer of the headbox.
Another aspect of the invention resides in a method for making chemically treated paper products. The method comprises the steps of: creating a first fiber slurry comprising water, cellulosic fibers, and an adsorbable chemical additive; creating a second fiber slurry that is substantially free of the adsorbable chemical additive; dewatering the first fiber slurry to remove unadsorbed chemical additive; redispersing the fibers in the first fiber slurry with fresh water; and forming a paper product using a layered headbox, the first fiber slurry supplied to a first headbox layer and the second fiber slurry supplied to a second headbox layer.
In another embodiment, a method for making a paper product comprises the steps of: creating a fiber slurry comprising water, cellulosic fibers and a first adsorbable chemical additive; dewatering the fiber slurry to a consistency of about 20 percent or greater; passing the dewatered fiber slurry through a disperser to mechanically work the fibers; diluting the fiber slurry with fresh water that is substantially free of the first chemical additive to a consistency of about 5 percent or less; adding a second adsorbable chemical additive comprising a debonding agent or a softening agent to the fiber slurry; dewatering the fiber slurry to a consistency of about 20 percent or greater; diluting the fiber slurry with fresh water that is substantially free of the second chemical additive to a consistency of about 5 percent or less; and forming a paper product from the fiber slurry. The first chemical additive may comprise, for example, a bonding agent to decrease the amount of lint from the product.
The present invention is particularly useful for adding chemical additives such as softening agents and debonding agents to the outer layer furnishes in a three layer paper product. In particular tissue products, for example, the center layer is adapted to provide strength development and control. The present invention allows the softening agents and debonding agents to be applied to the outer layers while minimizing contamination of the center strength layer.
Hence, another aspect of the invention resides in paper products formed from fibers that have been chemically treated to minimize the amount of residual, unadsorbed chemical additives in the process water. These paper products exhibit high chemical xe2x80x9cpurityxe2x80x9d on the fiber fraction that has been treated using the present method and offer the ability to achieve excellent chemical layer purity when using a stratified headbox and/or the ability to achieve fiber specific chemical treatment in papers made from blends of two or more fiber types. The term xe2x80x9cpaperxe2x80x9d is used herein to broadly include writing, printing, wrapping, sanitary, and industrial papers, newsprint, linerboard, tissue, napkins, wipers, towels, or the like.
The chemical additives that can be used in conjunction with the present invention include: dry strength aids, wet strength aids, softening agents, debonding agents, absorbency aids, sizing agents, dyes, optical brighteners, chemical tracers, opacifiers, dryer adhesive chemicals, and the like. Additional forms of chemical additives may include: pigments, emollients, humectants, viricides, bactericides, buffers, waxes, fluoropolymers, odor control materials and deodorants, zeolites, perfumes, debonders, vegetable and mineral oils, humectants, sizing agents, superabsorbents, surfactants, moisturizers, UV blockers, antibiotic agents, lotions, fungicides, preservatives, aloe-vera extract, vitamin E, or the like. Suitable chemical additives are adsorbable by the cellulosic papermaking fibers and are water soluble or water dispersible.
The term xe2x80x9csoftening agentxe2x80x9d refers to any chemical additive that can be incorporated into paper products such as tissue to provide improved tactile feel. These chemicals can also act as debonding agents or can act solely to improve the surface characteristics of tissue, such as by reducing the coefficient of friction between the tissue surface and the hand.
The term xe2x80x9cdebonding agentxe2x80x9d refers to any chemical that can be incorporated into paper products such as tissue to prevent or disrupt interfiber or intrafiber hydrogen bonding. Depending on the nature of the chemical, debonding agents may also act as softening agents. In contrast, the term xe2x80x9cbonding agentxe2x80x9d refers to any chemical that can be incorporated into tissue to increase or enhance the level of interfiber or intrafiber bonding in the sheet. The increased bonding can be either ionic, Hydrogen or covalent in nature.
The term xe2x80x9cdyexe2x80x9d refers to any chemical that can be incorporated into paper products, such as bathroom tissue, facial tissue, paper towels and napkins, to impart a color. Depending on the nature of the chemical, dyes may be classified as acid dyes, basic dyes, direct dyes, cellulose reactive dyes or pigments. All classifications are suitable for use in conjunction with the present invention.
The term xe2x80x9cwater solublexe2x80x9d refers to solids or liquids that will form a solution in water, and the term xe2x80x9cwater dispersiblexe2x80x9d refers to solids or liquids of colloidal size or larger that can be dispersed into an aqueous medium.
The method for applying chemical additives to papermaking fibers may be used in a wide variety of papermaking operations, including wet pressing and creped or uncreped throughdrying operations. By way of illustration, various tissue making processes are disclosed in U.S. Pat. No. 5,667,636 issued Sep. 16, 1997 to S. A. Engel et al.; and U.S. Pat. No. 5,607,551 issued Mar. 4, 1997 to T. E. Farrington, Jr. et al.; which are incorporated herein by reference.
The method may also be used in alternative processes, including: chemically pre-treating pulp in a pulp mill before a dry lap machine or crumb baler; adding chemical additives in sequence to reduce interactions; removing chemical additives from a fiber slurry (neutralizing anionic components, sizing or softening formulations) after a chemical additive has been added to facilitate the removal process; or the like.
Many fiber types may be used for the present invention including hardwood or softwoods, straw, flax, milkweed seed floss fibers, abaca, hemp, kenaf, bagasse, cotton, reed, and the like. All known papermaking fibers may be used, including bleached and unbleached fibers, fibers of natural origin (including wood fiber and other cellulosic fibers, cellulose derivatives, and chemically stiffened or crosslinked fibers), some component portion of synthetic fibers (synthetic papermaking fibers include certain forms of fibers made from polypropylene, acrylic, aramids, acetates, and the like), virgin and recovered or recycled fibers, hardwood and softwood, and fibers that have been mechanically pulped (e.g., groundwood), chemically pulped (including but not limited to the kraft and sulfite pulping processes), thermomechanically pulped, chemithermomechanically pulped, and the like. Mixtures of any subset of the above mentioned or related fiber classes may be used. The fibers can be prepared in a multiplicity of ways known to be advantageous in the art. Useful methods of preparing fibers include dispersion to impart curl and improved drying properties, such as disclosed in U.S. Pat. No. 5,348,620 issued Sep. 20, 1994 and U.S. Pat. No. 5,501,768 issued Mar. 26, 1996, both to M. A. Hermans et al. and U.S. Pat. No. 5,656,132 issued Aug. 12, 1997 to Farrington, Jr. et al.
Drying should be considered a means of further improving the substantivity of the chemical treatment. The two generally accepted methods of drying include flash drying and can drying. Flash drying is most common with bleached, chemi-thermo-mechanical pulp (BCTMP).
A single headbox or a plurality of headboxes may be used. The headbox or headboxes may be stratified to permit production of a multilayered structure from a single headbox jet in the formation of a web. In particular embodiments, the web is produced with a stratified or layered headbox to preferentially deposit shorter fibers on one side of the web for improved softness, with relatively longer fibers on the other side of the web or in an interior layer of a web having three or more layers. The web is desirably formed on an endless loop of foraminous forming fabric which permits drainage of the liquid and partial dewatering of the web. Multiple embryonic webs from multiple headboxes may be couched or mechanically or chemically joined in the moist state to create a single web having multiple layers.
Numerous features and advantages of the present invention will appear from the following description. In the description, reference is made to the accompanying drawings which illustrate preferred embodiments of the invention. Such embodiments do not represent the full scope of the invention. Reference should therefore be made to the claims herein for interpreting the full scope of the invention.
The general object of this invention is to provide paper products and a method for applying chemical additives to cellulosic fibers. More particularly, this invention relates to a method for applying chemical additives to cellulosic fibers used to make bathroom tissue.
Another object of this invention if to provide a method of adding a dye to cellulosic fibers at a location separate and distinct from the paper making equipment.
A further object of this invention is to provide a method for applying one or more chemical additives to cellulosic fibers at a location where the unadsorbed chemical additives can be removed without contaminating process water.
Still another object of this invention is to provide a method of dying cellulosic fibers to alter the color of the fibers before they are directed to a paper making machine.
Still further, an object of this invention is to provide a method for applying a chemical additive to cellulosic fibers which is economical and efficient.
Others objects and advantages of the present invention will become more apparent to those skilled in the art in view of the following description and the accompanying drawings.